1. Field of the Invention
The present invention relates to an optical timing extraction circuit for use in the field of optical communication.
2. Description of the Prior Art
In recent years, as optical communication techniques has been developed in response to a demand for further speeding up of the performance of communication networks, optical transmission whose transmission speed amounts to tens of Gb/s is being realized. In this case, a timing extracting circuit is indispensable for the transmission system. However, it is difficult to realize such a circuit in a transmission speed range of over several tens of Gb/s with electronic circuits, and a clock extracting technique using an optical circuit becomes necessary. Conventionally, as such, an optical timing extraction circuit using a narrow bandwidth optical filter, such as a Fabry-Perot filter, as a clock extracting tank, was proposed in a paper entitled "All-optical Timing Extraction Circuit", Jinno et al, Material of Institute of Electronics, Information and Communication Engineers, Optical Communication System Seminar, December 1988, pages 68 through 74, in which its free spectral range substantially coincides with the fundamental frequency of the transmitted signal.
However, in a conventional optical timing extraction circuit, the transmission line signal from which the clock signal can be extracted is restricted to an RZ signal, and it cannot be done for an NRZ signal. Therefore, with the conventional optical clock extracting circuit, there has been a problem to be solved regarding the form of the optical signal from which the clock signal is to be extracted.